1. Field of the Invention
This invention generally relates to recovering precious metals from refractory ores. It is particularly concerned with recovering gold and silver from those refractory sulfide ores that have been concentrated by one or more preceding ore processing steps (e.g., flotation cell operations, gravity separations, etc.) and are further characterized by their low precious metal values.
2. Description of the Prior Art
Precious metals are often associated with various sulfide minerals. These minerals are usually characterized as "refractory ores" when their precious metal values are occluded in a metallic sulfide host material. Gold, for example, is often found in the form of finely disseminated sub-microscopic particles that are occluded within a refractory sulfide host of pyrite or arsenopyrite. Consequently, the gold-encapsulating sulfide host material must be at least partially oxidized in order to make the ore's gold component more amenable to subsequent recovery processes wherein the sub-microscopic gold particles are exposed to a leaching agent such as cyanide.
Various sulfide oxidizing "pre-treatments" (i.e., treatments that take place prior to leaching the ore's gold component) have been developed. The most commonly used pre-treatments involve roasting, pressure oxidation and/or bacterial oxidation processes. Unfortunately, each of these processes has certain drawbacks. For example, roasting requires that the temperature of the refractory sulfide ore be raised to levels (e.g., approximately 650 C) that will burn off its sulfide component. Attainment of such temperatures implies high fuel costs. Moreover, in order to obtain autothermal combustion, roasting pre-treatments require that the sulfide component of the refractory sulfide ore be more than about 20 weight percent. Such ores also must have low moisture levels (e.g., the ore should have less than 8% weight percent water). Violation of either of these constricts raises the heat requirements for (and hence, the fuel expense of) roasting operations. Fuel expenses are not, however, the most important drawback associated with refractory sulfide ore roasting. At 650 C roasting temperatures, sulphur components of a refractory sulfide ore react with the surrounding air's oxygen to form various noxious, sulfur oxide gases (e.g., SO.sub.2 and SO.sub.3). In earlier times, these gases were simply vented to the atmosphere. More and more stringent governmental regulations have, however, restricted such venting practices to a point where most refractory sulfide ore roasting operations have been discontinued.
Pressure oxidation processes employ high purity oxygen, at high temperatures and at high pressures, to oxidize the sulfur components of refractory ores. Aside from the venting problems that are associated with this technology, the high temperatures, high pressures and high oxygen purity requirements of these processes, as well as their need for very expensive, corrosion-resistant autoclave equipment, have prohibited their more extended use, especially for pre-treatment of lower grade refractory ores.
Bacterial oxidation of refractory ores are being more and more widely used, but they are not without their drawbacks. Such processes generally fall into two categories: closed tank biooxidation and heap biooxidation. Each of these processes takes advantage of the fact that certain microorganisms are capable of oxidizing certain metal sulfide materials. For example, various bacteria have been used to oxidize the iron sulfide component of refractory ores. The use of closed tank biooxidation processes is, however, generally limited to use upon those refractory ores having relatively high precious metal value concentrations. Closed tank biooxidation processes also tend to become prohibitively expensive when the refractory ore being treated has a relatively high sulfide concentration. In effect, high sulfide concentrations in refractory ores tends to drive the air supply, cooling, and power input requirements of closed tank biooxidation processes to unacceptable levels. In general, such processes can not be economically justified to pre-treat those ores where the ratio of gold, or precious metal equivalent (in g/t), divided by its sulfur content (in %) is smaller than about 0.7.
The other widely used bacterial oxidation process for treating refractory sulphide ores is open air, heap bioleaching. It begins by breeding a bacterial culture in a liquid medium. The resulting bacteria suspension is then used to agglomerate an unconcentrated form of the ore. The agglomerated ore is stacked (on an appropriate pad system) in the open air and sprayed with the bacteria suspension. Under such conditions, rather long periods of time (e.g., from about 180 to about 600 days) are needed to oxidize the refractory ore's sulfide component. These long process time periods imply high production costs. Eventually, however, the resulting biooxidized refractory ore can be gathered, mixed with lime in order to raise its pH, and then restacked in order to prepare it for conventional hydrometallurgical treatments such as cyanide heap leaching.
Aside from the long periods of time needed to practice them, heap bioleaching processes also have certain technical drawbacks. These drawbacks often follow from the fact that finely ground clay and/or refractory sulfide materials tend to migrate through a heap and plug its channels of air and liquid flow. This results in pudding, channeling, nutrient, carbon dioxide and/or oxygen-starvation, as well as uneven biooxidant distributions. Blocked heap channels have particularly debilitating effects on sulfide-digesting bacteria because these bacteria require large amounts of oxygen to grow and biooxidize the iron sulfide component of such ores. Air flow is also needed to dissipate the heat generated by the exothermic biooxidation reactions that are carried out by sulfide digesting bacteria.
Various closed tank processes, and open air heap biodigesting processes, have been the subject of a number of patents. For example, South African Patent 90/2244 teaches a closed tank bioleaching process for treatment of refractory sulphide ores. This process includes the steps of making a slurry from a refractory ore, subjecting the slurry to the biological oxidation action of certain Thiobacillus ferrooxidans species, separating the solid component of the slurry, and then recovering the precious metal from said solid component by, for example, cyanidation procedures.
U.S. Pat. No. 5,246,486 teaches a pre-treatment process based upon biooxidation of a sulfide component of a refractory ore. The process begins by coating refractory sulfide ore particles with an inoculate of a bacteria that is capable of attacking the sulfide component of such an ore. After various other treatments, a heap is constructed from these particles and exposed to the action of a cyanide leaching solution.
U.S. Pat. No. 5,143,543 teaches an improved method of mixing biological conversion components (e.g., nutrients and oxygen) into a biomass. To this end, a portion of a biomass is withdrawn from a reaction tank and sent to an injection zone where the conversion components are injected into a portion of biomass previously withdrawn from the reactor. The resulting mixture is then sent to a static mixer where it is combined with other streams. The resulting material is then returned to the reaction tank.
U.S. Pat. No. 5,021,088 teaches a process for pre-treating gold-bearing, carbonaceous or carbonaceous pyretic ores with one or more heterotrophic microorganisms in order to consume the ore's carbon component. The resulting ore is then colonized with one or microorganisms whose sulfide digestion processes serve to further enhance the ore's susceptibility to subsequent cyanidation processes.
U.S. Pat. No. 4,530,763 teaches a method for removing a metal contaminant from a waste fluid by a process that begins by incubating a bacteria that is capable of attaching to a particular type of metal contaminant. A suspension of the bacteria is placed in a closed tank that is equipped with porous support members suitable for promoting bacteria growth thereon. After the bacteria have had an opportunity to attach themselves to the porous support members, the bacterial medium is removed from the tank. A waste fluid containing the targeted metal contaminant is then introduced into the tank and the porous support members are slowly moved through the waste fluid to allow the bacteria on these support members to attach themselves to the metal contaminant component of the waste fluid. The resulting bacteria/metal contaminant is then separated from the porous support material.
U.S. Pat. No. 5,573,575 teaches a process whereby differences in the adhering qualities of refractory ore particles of different sizes are employed to enhance the overall recovery efficiencies of an open heap leaching process. The first step in the disclosed process is to crush the refractory ore and separate it into a fine particle component and a coarse particle component. The coarse particle component is formed into a heap. The fine particle component is made into a large particle concentrate material that is then added to the coarse particle component heap. The resulting coarse particle/large particle concentrate mixture is thereafter exposed to a heap biooxidation treatment.
U.S. Pat. No. 5,766,930 teaches a process for biotreating a solid material such as an ore in order to remove certain undesired compounds such as sulfides. The process employs a nonstirred bioreactor for this purpose. Using this process, the surface of a plurality of coarse substrates is coated with a solid material to be biotreated to form a plurality of coated coarse substrates. The coarse substrates have average particle sizes greater than about 0.3 cm and the ore particles to be biotreated have average particle size less than about 250 .mu.m. A nonstirred surface reactor is then formed by stacking the plurality of coated coarse substrates into a heap--or placing the plurality of coated coarse substrates in a tank so that the void volume of the reactor is greater than or equal to about 25 percent. In either case, the ore is inoculated with a microorganism capable of degrading the undesired compound in that ore, and the resulting material is then biotreated in a surface bioreactor until the desired compound in the solid material is degraded to some desired level.
U.S. Pat. No. 5,873,927 teaches a biotreating process wherein a metal-containing refractory sulfide ore stream is split into a first portion and a second portion. The first portion is partially biodigested by a sulfide-digesting microorganism in a biooxidation reactor where the microorganism is acclimated to the sulfide "diet" provided by that particular sulfide-containing ore. The partially digested ore is then combined with the second portion. The resulting mixture is then dewatered, and, preferably, agglomerated, and then biooxidized. Thereafter, it is subjected to a lixiviation process.
These prior art apparatus and processes often suffer from the disadvantage of being prohibitively expensive when they are used upon low grade ores in general--and especially those low grade ores that emanate from relatively small ore bodies. Indeed, there are large amounts of identified low grade refractory ores, as well as stocks of mined ore, that have been set aside because they cannot be processed economically using current precious metal recovery technologies. It is therefore an object of the present invention to provide biooxidation pre-treatment processes that can render such ores amenable to lixivation at economically acceptable costs.